Small form factor betavoltaic battery for medical implants

ABSTRACT

A betavoltaic power source. The power source comprises a source of beta particles, one or more regions for collecting the beta particles and for generating electron hole pairs responsive thereto, and a secondary power source charged by a current developed by the electron hole pairs.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. 119(e) to theprovisional patent application filed on Jun. 13, 2013 and assignedapplication No. 61/834,671. This provisional patent application isincorporated in its entirety herein.

FIELD OF THE INVENTION

The present invention applies to small form factor betavoltaic batteriesespecially for use with medical implants and in other applications wherea small form factor is desired.

BACKGROUND OF THE INVENTION

Efforts aimed at miniaturization have always been a strong componentassociated with the advancements in the medical device/implant industry.In the 21^(st) century, this trend continues to be ever-present with adrive to scale down devices from a cubic centimeter range down to acubic millimeter range. Smaller scale medical devices demonstrate greatpotential for reducing healthcare costs and mitigating trauma associatedwith invasive implant surgeries, while concomitantly improving bothpost-operative medical evaluations and convalescence periods.

The benefits of miniaturization are demonstrated with the emergence of anew class of cardiac pacemaker devices that are small enough to beinserted directly into a patient's right ventricle. The size reductionof these pacemakers and other medical devices is limited, in part, bythe power sources that fuel the device's operation. Pacemaker batteriestypically consume up to 40-80% of the device's volume. Consistent withthis notion, further significant reductions in the scale of medicaldevices have been limited by the power sources themselves. While devicecomponents and electronic circuitry can be reduced to ever morediminutive dimensions, battery technology has traditionally remainedlimited to cubic centimeter dimensions arising as a consequence ofexponential losses in energy density and capacity as batteries approachcubic millimeter scales. However, recent advancements in bothelectronics and battery technologies have led to reduced system powerdemands and higher power outputs from more diminutive power solutions.

Recent academic teams have laid the groundwork for an implantable cubicmillimeter-scale device that can measure a glaucoma patient'sintra-ocular eye pressure. This tiny device comprises a CMOSmicrocontroller that contains an on-board radio transmitter and sensorwith cubic millimeter dimensions. The device operates at ultra-low powerlevels, e.g. picowatt power range during sleep cycles and atapproximately microwatt power range during operational periods. Due tothe extremely small dimensions of the device, it can be directlyimplanted within a patient's eye. The device can measure/recordintra-ocular pressure throughout the day and radio-transmit processeddata at periodic intervals to an external device for analysis.

This millimeter scale device utilizes an exceedingly small-sized solarcell to trickle-charge a millimeter scale LiPON (lithium phosphorousoxynitride) battery to provide a rechargeable power source. Such LiPONbatteries are available from Cymbet Corporation of Elk River Minn., amanufacturer of small thin film, solid state secondary batteries. TheCymbet LiPON battery has approximately a 1 to 50 microamp-hour capacity,and the duty cycle of the device allows it to operate indefinitely aslong as the solar cell provides an average of 10 nanoamps of current tothe LiPON battery.

This configuration is well-adapted to an ocular implant where visiblespectrum light energy is easily accessible and can be transmittedthrough the eye thereby providing the solar cell with the necessarytrickle-charge power for the LiPON battery. However, such a solution isnot suitable for implants that are not accessible to visible light,thereby rendering the solar cell component moot. An example of such ascenario might be a sensor embedded within a tumor for measuringpressure changes associated with tumor growth.

It should be noted that the LiPON battery can at best provide a maximumof 28 days of operation for implants in a stand-alone operation; this isfar too short for most implantable devices that are not accessible tovisible light sources. Clearly, a long-term (light-independent)trickle-charging source is required for maintaining the LiPON batteryand subsequently facilitating operational effectiveness of such smallscale sensor systems operating with such a LiPON battery.

Unfortunately, chemical batteries with less than cubic centimeterdimensions have a less than optimal energy density. The optimal energydensity for a lithium iodide battery in conventional pacemakers isapproximately 1 watt-hour per cubic centimeter with a volume greaterthan about 3.0 cubic centimeters. To supply sufficient power toshrinking medical devices, an energy density of greater than about 1watt-hour per cubic centimeter is desirable for battery volumes ofapproximately 0.5 cubic centimeters or less. In battery volumes of 0.1cubic centimeters, a 10 watt-hour per cubic centimeter energy densitywould be highly desirable due to the loss of energy capacity in such asmall volume (i.e. 1 watt-hour of capacity in a tenth of a cubiccentimeter).

Furthermore, the battery needs to provide power in ranges from nanoampsto milliamps to accommodate duty cycle power requirements of medicaldevice electronics. For example, wireless signals (to transmit thesensed values to an external device) require higher power bursts forshort durations while the microcontroller's sleep power provides a low,but continuous drain. An ideal medical implant power source will have ahigh energy density that is robust under a wide-range of power drainswhile having a diminutive form factor. Unfortunately, currentlyavailable betavoltaic power sources with less than 1 cubic centimeterdimensions will have only nano-watt to low-microwatt power outputsfalling short of the required, or at least desired, higher power outputnecessary in some medical implant functions (e.g. radio telemetry,defibrillation etc.).

Historically, betavoltaic power sources for medical device implants arenot a new concept as they have a demonstrated potential for high energydensities well beyond conventional chemical batteries. Betavoltaic powersources do, however, suffer from low power densities and radiationshielding requirements. In the early 1970's, a group of researchers atDonald W. Douglas Laboratories of Richland, Wash. (led by Dr. LarryOlsen) invoked a promethium-147 radio-isotope to fuel a betavoltaicpower source (also referred to as a Betacel) for energizing a cardiacpacemaker, which was successfully implanted in over 100 patients.Although the Betacel's size approximated 1.0 cubic inch due to shieldingrequirements incurred from an associated gamma radiation emittingpromethium-146 component, the successful implementation of thistechnology demonstrated the feasibility of betavoltaics for use withinmedical implants.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing and other features of this invention will be apparent fromthe following more particular description of the invention, asillustrated in the accompanying drawings, in which like referencecharacters refer to the same parts throughout the different figures.

FIG. 1 illustrates a triple junction for use with the present invention.

FIG. 2 illustrates a bi-directional betavoltaic cell

FIG. 3 illustrates use of a betavoltaic battery in an implantabledevice.

FIG. 4 illustrates a betavoltaic battery of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the teachings of the various embodiments of this invention, miniaturebetavoltaic power sources (with dimensions ranging from about a cubicmillimeter to approximately 0.5 cubic centimeters) are constructed withenergy densities approximately ranging from about 1 watt-hour per cubiccentimeter to about 500 watt-hours per cubic centimeter. The energydensity for a betavoltaic power source is calculated by integrating thebetavoltaic device's power density over the medical device's useful life(e.g. 10 years for pacemakers). Betavoltaic power sources can beconstructed in cubic millimeter volume spaces without affecting theenergy density; this is unlike conventional chemical battery technologywhere practical limitations exist in constructing micro-scaled cathodes,anodes, and liquid electrolyte volumes without incurring losses inenergy density.

In the case of promethium the inventor has discovered that the removalof the Pm-146 component is an important factor in reducing the shieldingrequirements for the betavoltaic battery. The gamma of Pm-146 is veryhigh energy and is difficult to shield. The resulting shieldrequirements after removing the Pm-146 component are considerably lessthan the original BetaCel, as described above, that had the Pm-146component with the Pm-147 component.

Tritium beta flux can be shielded with a thin sheet of paper but tritiumbetavoltaic power sources of the prior art have been too low for use asa power source for medical electronics. It is only with the thinning ofInGaP cells, new high tritium density metal tritide films and/or the useof enhanced surface area semiconductors, as described herein or in thereferenced commonly-owned patent applications, that the inventor hasbeen able to approach energy densities of 1 watt-hour/cm^3 or greaterand power densities in the range of 10's to 100's of microwatts/cm^3.

Betavoltaic power sources are typically comprised of a beta-emittingradioisotope affixed directly onto a semiconductor collector that ispackaged in a container, which provides radiation shielding to levelsappropriate for the intended application. The semiconductor collectormaterial is similar to a solar cell and can easily be miniaturized tomicro-scales without exhibiting losses in electrical properties.Furthermore, radioisotopes such as tritium (H-3), promethium-147(Pm-147), and nickel-63 (Ni-63) can all be incorporated into metallicforms, which can be similarly scaled to the semiconductor collector'sdimensions.

In one embodiment of this invention, a betavoltaic power source with avolumetric dimension of approximately 0.5 cubic centimeters or lessoutputs power in the nano-watts to microwatts range and can providenominal power in standby or active modes to medical implant circuitry.

In another embodiment of this invention, a betavoltaic power source witha volumetric dimension of approximately 0.5 cubic centimeters or lessoutputs power in the nano-watts to microwatts level and can providenominal power in standby or active modes to the medical implantcircuitry while utilizing a portion of its generated power to tricklecharge a secondary battery and/or a capacitor and/or another energystorage device. The secondary battery and/or capacitor and/or otherenergy storage device is capable of modulating interim power that attimes may be higher than the standalone betavoltaic power supply (e.g.power bursts for radio telemetry, defibrillation etc.).

In yet another embodiment of this invention, a betavoltaic power sourcewith a volumetric dimension of approximately 0.5 cubic centimeters orless outputs power in the nano-watts to microwatts level and can providenominal power in standby or active modes to the medical implantcircuitry while utilizing a portion of its power to trickle charge asecondary battery and/or a capacitor and/or another energy storagedevice with volumetric dimensions of approximately 0.5 cubic centimetersor less. The secondary battery and/or capacitor and/or other energystorage device is capable of modulating interim power that may be higherthan the standalone betavoltaic's power supply (e.g. power bursts forradio telemetry, defibrillation etc.).

In one embodiment of this invention, a Pm-147-based betavoltaic powersource with a volume that is less than approximately 0.5 cubiccentimeter is constructed. The Pm-147 betavoltaic can be hermeticallysealed within a cylindrical form factor (or another form factorappropriate for the intended application) that, in turn, allowscurrently available medical device delivery systems (e.g. catheters,stent delivery systems, syringe, etc.) to implant these device in vivo.The seal may comprise a bio-inert stainless steel package with a seamsealer, resistance welder, ultrasonic welder or laser-welder. Thebio-inert package can be made with ceramic and/or metal materials knownin the art.

The Pm-147 isotope is made free of Pm-146 via current methods known inthe art (e.g., see U.S. Pat. No. 7,435,399) allowing for modestradiation shielding requirements (e.g. use of a biocompatiblemetals/materials sufficient for shielding). This is in stark contrast tothe original Betacel Pm-147, as described above, betavoltaic powersupply for cardiac pacemakers that required considerable shielding dueto the concomitant Pm-146 component.

In addition, the Pm-147 can be made into a bidirectional radiationsource using methods known in the art. The semiconductor collector (e.g.a pn junction) may be constructed from silicon material or may fromother semiconductor materials known in the art (GaAs, GaP, InGaP, GaN,SiC etc).

In one example, the semiconductor collector comprises a type III-Vmaterial such as GaAs. GaAs has a relatively high diffusion length forits minority carriers but still requires multiple junctions to be grownin order to capture the majority of the electron-hole pairs (EHPs)generated via the beta particles impinging on the semiconductorcollector.

FIG. 1 illustrates a triple-junction device (three pn regions 30, 32 and34) where EHPs may be collected throughout the collector's volume. Eachjunction can be made between 15-20 microns thick or in appropriatedimensions that optimize the collection efficiency. It is estimated thatPm-147 beta particles can travel approximately 60 microns or more into aGaAs semiconductor. Optimization of the junction thickness and thenumber of junctions can be made for different semiconductor collectormaterial. The vertical dashed line in FIG. 1 denotes the boundary of a pand an n region; the larger region illustrated in FIG. 1 can be eitherthe p region or the n region.

In another embodiment of a Pm-147 betavoltaic, two triple junction GaAscollectors are constructed with a Pm-147 bidirectional source in themiddle forming a unit. Both triple junctions are placed in intimatecontact with Pm-147 source and are electrically connected in parallel.The GaAs triple junction collector has circular dimensions with adiameter of about 0.4 centimeters and a thickness of about 120 microns.Each p/n region forms a junction so three p/n regions form a triplejunction. This technique is used in solar cells today in order tocapture different wavelengths of light based on their penetration depthinto solar cells.

FIG. 2 illustrates such a device with a source 40 between triplejunctions 42 and 44. Each junction is approximately 15-20 microns thickand the Pm-147 bidirectional source is approximately 6 milligrams persquare centimeter at 400 Curies per gram or 2.4 Curies per squarecentimeter. The combined bidirectional unit of FIG. 2 comprises 1.0square centimeter and yields an open circuit voltage of 2.4 volts and ashort circuit current of 14 microamps generating approximately 27microwatts of power at beginning-of-life (BOL) with a fill-factor ofapproximately 80 percent. The half-life of Pm-147 is approximately 2.62years and it should be noted that many medical devices have a usefullifetime of 3-5 years thereby permitting use of a Pm-147 device.

Note that the thickness of the individual regions within the triplejunctions 42 and 44 are greater at increasing distance from the source40 to account for the exponential decay of the beta particle flux. Thearrowheads represent the beta particle flux. Generally, the thickness ofthe individual regions is selected so that about the same current isproduced within each individual region. The individual regions (or cellssince each region produces a current) within the triple junction 42 arein series and the individual regions within the trip junction 44 are inseries. But the triple junctions 42 may be connected in series or inparallel with the triple junctions 44. The FIG. 2 embodiment is shown inan elongated representation; in another embodiment the source 40 and thejunctions 42 and 44 are each shorter and thus the combination is moredisc-like than the tubular representation of FIG. 2.

The power density for this betavoltaic device 40 is approximately 5milliwatts per cubic centimeter with an energy density of approximately125 Watt-hours per cubic centimeter when integrated over two half-lives(5.24 years). In order to modulate the power outputs from the Pm-147betavoltaic source, two bidirectional units may be placed in series inorder to yield an open circuit voltage of 4.8 Volts with a short circuitcurrent of 14 microamps at BOL. The two bidirectional units may beconnected to a LiPON battery, and/or capacitor and/or energy storagedevice configuration in order to receive a trickle charge andsubsequently discharge/modulate higher power over a duty cycle oron-demand. By combining one or more LiPON batteries within a smallmillimeter scale volume, the power emanating from the Pm-147 betavoltaicunits can be modulated freely from the nanoamp up to the maximum currentoutput (e.g. Amp-seconds). The approximate dimensions for this assembly,comprising two bidirectional Pm-147 sources, four betavoltaiccollectors, and the LiPON batteries, is 0.05 cubic centimeter.

Rechargeable thin-film LiPON batteries may be purchased from CymbetCorporation of Elk River, Minn. or Infinite Power Solutions fromLittleton, Colo. LiPON batteries are robust over 10,000 charge/dischargecycles and have a low discharge rate of 3-6% even at human bodytemperatures. LiPON batteries can be discharged at rates of 10 C(capacity) and in short bursts up to 40 C without deleterious effectsupon its capacity or performance. For instance, in the case of a batterywith 50 microamp-hour capacity it is possible to discharge at 1 C. Thiswould be a 1 hour discharge at a rate of 50 microamps continuous. Or itcan be discharged at 40 C for a short period, resulting in a rate of 2milliamps.

Cymbet Corporation's LiPON batteries can be fabricated with form factorswell within cubic millimeter dimensions and with energy capacities of1-50 microamp-hours. Infinite Power Solutions' batteries have capacitiesin the sub-milliamp-hour range, but they demonstrate a largerform-factor with linear dimensions measuring in the centimeter range andthicknesses of 180 microns. However, they are malleable enough toconform to cylindrical shapes. It should be noted that custom LiPONbatteries with various dimensions and capacities can also be fabricatedas desired.

In another embodiment, a tritium-based betavoltaic collector cellinvolving a III-V semiconductor, such as the structures described in thecommonly-owned U.S. Pat. No. 8,487,507, entitled Tritium DirectConversion Semiconductor Device and incorporated herein by reference, isutilized. In one embodiment the dimensions are approximately 0.5 cm×0.5cm or smaller per betavoltaic collector layer. Other embodiments mayhave larger dimensions. Additionally, the betavoltaic layers may beconstructed with an approximate 0.5 cm diameter or smaller in order tooptimally fit into a cylindrical medical devices that can be implantedvia a catheter-like delivery system. The thickness of the betavoltaiccell is approximately 10-25 microns or less with a tritium metal tritidelayer (e.g. titanium, scandium, magnesium, lithium, palladium, etc.)having a thickness approximately ranging from less than 100 nanometersto greater than 1.0 micron, that is placed in contact with the activesurface of the betavoltaic cell.

This is a thin III-V betavoltaic cell that utilizes anepitaxial-lift-off process or etching away of the substrate coupled witha back metallization to offer conduction and support of the cell.Additionally, the thin III-V betavoltaic cell may use a metal tritidelayer as back metallization via the deposition of candidate metaltritide forming metals or combination of metals as listed above. Thesemetals may be deposited through evaporation, sputtering or other methodsknown in the art. The metal tritide back metal offers support to thethin betavoltaic cell and an additional source of beta-flux through theback-side of the betavoltaic cell; thereby increasing electron-hole pairgeneration and the overall production of electrical power. It should benoted that the metal tritide metal may in some instance be insulatingdue to the tritium in the metal matrix. In such cases, a portion of theback side metal may be a conductive non-hydriding forming metal so as toprovide electrical conduction for the betavoltaic cell. In order toachieve reasonable power densities the cells are stacked in series andparallel with the aid of flexible circuit cards or other interposersbetween some of the layers to make parallel connections or even seriesconnections.

The betavoltaic cell may also have thicknesses ranging fromapproximately 25 microns to 625 microns. For thicker betavoltaic cells(e.g. 25 microns to 625 microns or greater), the inventors propose touse a surface enhancement technique to raise the power density/energydensity. Although, surface enhancement techniques may also be utilizedin thinner betavoltiac cells (e.g. 25 microns or thinner).

Another approach that may be used is to thin the back surfaces of thebetavoltaic semiconductor substrates by etching, polishing, grinding,and/or other lapidary techniques known in the art. The scandium,titanium, magnesium, palladium, lithium, or other suitable tritideforming layer may be metallurgically placed on top of the betavoltaiccell by directly depositing on top of the betavoltaic cell's active areathrough methods known in the art (e.g. evaporation, electro-deposition,sputtering, etc.). Alternatively, the metal tritide layer may bedeposited on a separate thin substrate (i.e. ˜25 microns to ˜500microns) that is mechanically connected to the betavoltaic cell's activearea via pressure, epoxy, or spot welding.

In one embodiment the inventor prefers to have the tritide metal layerevaporated onto the cell's active area rather than to have a separatesubstrate. This allows the formation of a single monolithic piece thatincludes both the metal tritide and the betavoltaic collector. Inanother embodiment a betavoltaic cell comprises two separate components(betavoltaic collector and tritium metal tritide layer/foil), butgenerally this embodiment occupies more volume and is lower in bothpower and energy density.

The metal tritide is formed by exposure to tritium gas at pressuresapproximately ranging from less than 0.01 to greater than 20 Bar andtemperatures ranging approximately room temperature to 600° C. fordurations ranging minutes to days. A cap layer of palladium ranging fromapproximately 1.0 nanometer to 500.0 nanometers may be deposited over ascandium, titanium, magnesium, lithium, or other suitable metal in orderto reduce the tritium loading temperature and stabilize the tritiumwithin the metal matrix after the tritide has been formed. The palladiumcap layer functions primarily as a catalyst and serves to provide for anexpedited rate of reaction for inducing the process of tritiation;palladium has an additional benefit for the tritiation process in thatit can facilitate tritium loading of a metal tritide at significantlylower temperatures and pressures compared to processing effortsconducted in the absence of palladium. This subsequent increase in thekinetics of the tritiation process induced by the palladium cap layerdoes not alter the ultimate functionality of the betavoltaic cell, andit is usually deposited directly upon un-passivated surfaces (surfacescontaining no oxide barriers to tritiation) of metal hydride storagelayers (e.g. scandium, titanium, magnesium, lithium, or other suitablemetal). The palladium layer is typically laid down in a vacuum/inert gasatmosphere process, in order to eliminate oxygen contamination and isdeposited via any of the metal deposition techniques described elsewhereherein. It should also be noted that the metal tritide layer may also beformed via an in-situ evaporation of the metal in the presence ofgaseous tritium.

The average current produced in a type III-V semiconductor in thepresence of the metal tritide is approximately 1.0 to 6.0 nanoamps persquare millimeter or 100-600 nanoamps per square centimeter. The opencircuit voltage for a type III-V structure ranges between 0.4 Volts to1.2 Volts.

Betavoltaic cell layers may be stacked vertically and configured inseries or parallel utilizing through-vias as power lead contacts acrossbetavoltaic cell layers or by using current-channeling interposers (e.g.flexible circuit cards or yttria-stabilzed zirconia dielectricmaterials) in between betavoltaic cells or groups of cells. It should benoted that various stacking configurations (serial, parallel,combination) produce different voltage and current outputs from thebetavoltaic composite device.

In one example, a tritium betavoltaic device is comprised of 10 micronthick betavoltaic cell layers with through-vias for electricalconnections. Each betavoltaic cell consists of a type III-Vsemiconductor material as described in the commonly-owned U.S. Pat. No.8,487,507 and entitled Tritium Direct Conversion Semiconductor Deviceand a tritium metal tritide deposited on its surface producing ashort-circuit current of about 300 nanoamps per square centimeter at BOL(beginning of life) and a voltage of 0.9 volts per cell layer with afill-factor of approximately 80%. The composite betavoltaic device canbe made with a variety of form factors ranging from cubic millimetervolumes or greater and varying geometries (e.g. cylindrical). The powerdensity is approximately 216 microwatts per cubic centimeter at BOL and15 Watt-hours per cubic centimeter when integrated over one tritiumhalf-life (12.3 years).

The application sets forth example dimensions for a betavoltaic sourceand its constituent elements. However, variations from the reciteddimensions provide useful sources with desirable operating properties.For example, a 150 micron thick cell may still be useful in certainapplications. For example, a commonly-owned provisional patentapplication assigned application No. 61/838,692 filed on Jun. 23, 2014describes enhanced surface area cells. In this case, the cell may be 150microns thick but it may have an effective area of 10× due tovalleys/trenches of the enhanced surfaces as described in theaforementioned provisional application, which is incorporated herein.

Similar to the Pm-147 betavoltaic embodiment, a 1-50 microamp-hour LiPONsecondary thin-film battery may be connected to the composite tritiumbetavoltaic device in order to modulate freely from nanoamp to milliampranges. Furthermore, other secondary battery, capacitor and/or energysystems may be utilized. This hybrid betavoltaic comprised of tritiumbetavoltaic layers that trickle charge a LiPON battery and/or capacitorand/or energy storage systems can be used to power leadless pacemakersrequiring average continuous power draw of approximately 5-30 microwattsfor over 12 years within a volume of approximately 0.1 cubiccentimeters. This represents a factor-of-ten reduction in volume beingapproximately one tenth the size of the smallest leadless pacemakerbatteries currently in existence today without sacrificing energydensity, capacity, or power output performance.

The application sets forth example dimensions for various betavoltaicsources and their constituent elements. However, variations from therecited dimensions provide useful sources with desirable operatingproperties. For example, a hybrid betavoltaic power source with 0.5cubic centimeters or greater may be desired in certain applications. Forinstance, a small betavoltaic source with secondary energy systems thatmay be larger than 0.5 cubic centimeters are useful in certainapplications.

In another example, this composite tritium betavoltaic's form factor maybe constructed with dimensions of 1.5 mm×2.0 mm×0.15 mm producing 0.1microwatts of power at BOL and can easily trickle-charge a 1.0microamp-hour LiPON battery measuring 1.5 mm×1.3 mm×0.15 mm. This smallform factor hybrid tritium betavoltaic can supply an ultra-low powermicrocontroller with a pressure sensor such as the implantable cubicmillimeter ocular-implant wireless pressure sensor for glaucoma patientsdescribed above.

The hybrid tritium betavoltaic may be physically bonded to the cubicmillimeter wireless pressure sensor comprising a microcontroller,memory, analog-digital converter, pressure sensor, and wirelesstransceiver to achieve a cubic millimeter scale device structure.

FIG. 3 illustrates such a device comprising a hybrid tritium betavoltaic50, a wireless transceiver 52 for sending a receiving signals from anexternal device, an ND and/or D/A converter 54, a processor and memoryelements 56, a chargeable power source 58, and a MEMS pressure sensor60. In one embodiment the structure of FIG. 3 is about 0.5 mm×2 mm×1.5mm.

The hybrid tritium betavoltaic source supplies power and energy capacityfor operating in any area of the body for over a decade without the needfor light collected via a solar cell. As a note, the invention is notlimited to the cubic millimeter volume set forth in FIG. 3 and can bemade using other ultra-low power cubic millimeter devices known in theart. Applications are also not limited to pressure and temperaturesensing but can make use of other ultra-low power sensing devices. Inaddition, it can be used for therapeutic purposes such as dispensingmedication from an on-chip dispenser that can be implanted in strategicmedical locations. It should be noted that cubic millimeter scaleultra-low power devices can vary in power consumption depending onconfiguration, duty cycle and sensor capabilities. However, the hybridtritium-based betavoltaic can meet varying current and voltagerequirements via stacking and electrical connection configurations.Similar betavoltaics with higher power in the range of microamps may bemade with other radioisotopes such as Pm-147.

The hybrid tritium betavoltaic coupled to the pressure sensor of FIG. 3can also be used in non-medical, external environments such as in meshsensor networks (e.g. Dust networks). It can also be used in conjunctionwith radioactive implantable seeds that irradiate tumors. This systemcan provide daily monitoring of a tumor and can provide information ofthe tumor's size change via changes in pressure. This approach can alsobe used with chemotherapy in-lieu of the radioactive seed. This type ofwireless sensor can also be used in stents to measure pressure/strainchanges that would actively monitor scar-tissue growth.

In another embodiment, the semiconductor collector's surface may beenhanced to increase the surface area of the collector. An increase insurface area provides an increase in power production ranging fromapproximately 2 to 100 times the power produced from a planar surfacebetavoltaic semiconductor collector. Semiconductor collector surfacestructures with increased surface area and power production aredescribed in a related and commonly owned provisional patent applicationfiled on Jun. 24, 2013 and assigned application No. 61/838,692.

In all embodiments, the radioisotope may be exchanged for otherbeta-emitting radioisotopes known in the art. Radioisotope selection canbe made to optimize power vs longevity and/or cost or availability. Forexample, Phosphorus-33 is a short lived beta-emitting radio-isotope witha half-life of 25.,3 days and has an energy spectrum that approximatesthat of Promethium-147. The advantage over Promethium-147 is that it isreadily available and less expensive and may be appropriate for certainshort-lived applications (e.g. short-lived medical implant sensors).

The increased surface area of the semiconductor collector may be usedwith or without the thin semiconductor epi-layers described in thispatent application and in the commonly-owned patent application assignedapplication Ser. No. 12/637,735 and filed on Dec. 14, 2009. Increasedsurface area semiconductor collectors can increase the energy density ofbetavoltaics by up to 100 times or greater over planar semiconductorsurfaces. Additionally, the enhanced surface area reduces the overallcost of betavoltaic devices by reducing the semiconductor area necessaryto power devices.

According to the certain embodiments of the present invention, itcomprises one or more beta sources coupled to betavoltaic collectors.The arrangement can be as simple as a two-dimensional beta source andbetavoltaic collector connected to a secondary battery, and/orcapacitor, and/or any other secondary energy storage unit. In anotherconfiguration, the beta sources/collectors can be configured in seriesor parallel to trickle charging the secondary cell or battery with thesecondary cell or battery for use during power bursts or modulations.The betavoltaic cell or battery can also utilize a portion of its powerto supply electrical systems without the aid of the secondary energystorage unit.

FIG. 4 illustrates a betavoltaic battery 70 comprising a source of betaparticles 72, regions 74 for collecting the beta particles, andsecondary power source 76 charged by the electron-hole pairs generatedwithin the region 74. In some embodiments, a palladium material layer 73is disposed proximate a first surface of the source of beta particles73, where the regions 74 for collecting the beta particles are proximatea second surface of the source of beta particles 72, the first andsecond surfaces on opposing sides of the source of beta particles 72.However, in other embodiments, the palladium material layer 73 isexcluded. In the illustrated embodiment the secondary power source isconnected to the regions 74 via conductors 78. In another embodiment(not illustrated) the secondary power source 76 is bonded to or inintimate contact with the regions 74 in lieu of utilizing the conductors78.

Note that palladium can also form a metal tritide. However, it does nothave as high a tritium content as, for example, a titanium tritide,scandium tritide, magnesium tritide, lithium tritide etc. However, thepalladium cap layer prevents oxidation of the titanium, scandium,magnesium or lithium metal below it since these metals will readilyoxidize and form an oxide barrier to tritium when exposed to air. Italso dissociates the T2 (tritium two) molecule to elemental T allowingit to diffuse and bond to the metal hydride below it (e.g. titanium,magnesium, lithium, scandium etc.). The palladium also helps keep thetritium in the metal hydride below because it prevents oxygen fromattacking the metal and releasing tritium species (e.g. tritium oxides,tritium metal oxides) into the surrounding environment. This makes thepalladium useful when handling the tritium sources post-tritiation. Itis helpful in the manufacturing process because of reduced tritiumcontamination.

While certain embodiments of the present invention have been shown anddescribed herein, such embodiments are provided by way of example only.Numerous variations, changes and substitutions will occur to those ofskill in the art without departing from the invention herein.Accordingly, it is intended that the invention be limited only by thespirit and scope of the appended claims.

What is claim is:
 1. A betavoltaic power source comprising a source ofbeta particles; one or more regions comprising semiconductor materialfor collecting the beta particles and for generating electron hole pairsresponsive thereto; and a secondary power source charged by a currentdeveloped by the electron hole pairs; wherein the betavoltaic powersource has a volumetric dimension of 0.5 cubic centimeters or less. 2.The betavoltaic power source of claim 1 wherein the source of betaparticles comprises tritium (H-3), promethium-147 (Pm-147), or nickel-63(Ni-63).
 3. The betavoltaic power source of claim 1 wherein the sourceof beta particles comprises a tritium metal tritide material.
 4. Thebetavoltaic power source of claim 1 wherein the secondary power sourcecomprises a secondary battery, a capacitor or an energy storage device.5. The betavoltaic power source of claim 1 wherein the one or moreregions comprise GaAs.
 6. The betavoltaic power source of claim 1wherein the source of beta particles is disposed between a first and asecond region for collecting the beta particles.
 7. The betavoltaicpower source of claim 1 wherein the secondary power source comprises aLiPON battery.
 8. The betavoltaic power source of claim 1 for use in animplantable medical sensor or a medical device.
 9. The betavoltaic powersource of claim 1 further comprising a palladium material layer disposedproximate a first surface of the source of beta particles, wherein theone or more regions are proximate a second surface of the source of betaparticles, the first and second surfaces on opposing sides of the sourceof beta particles.
 10. The betavoltaic power source of claim 1 whereinthe source of beta particles comprises a tritium metal tritide materiallayer.
 11. The betavoltaic power source of claim 10 wherein the tritiummetal tritide material layer comprises one or more of scandium,titanium, magnesium, palladium, and lithium.
 12. The betavoltaic powersource of claim 10 further comprising a palladium material layerdisposed on a surface of the tritium metal tritide material layer. 13.The betavoltaic power source of claim 10 wherein an amount of tritium inthe metal tritide material layer is selected to achieve a desired powerlevel or energy density.
 14. A betavoltaic power device comprising aplurality of betavoltaic power sources of claim
 1. 15. The betavoltaicpower source of claim 1 wherein one or more of the regions forcollecting the beta particles and for generating electron hole pairsresponsive thereto comprises a respective thickness based on a flux ofthe beta particles in each respective region.
 16. The betavoltaic powersource of claim 1 wherein the semiconductor material comprises at leastone p-n junction and wherein the semiconductor material is type III-Vsemiconductor material.
 17. The betavoltaic power source of claim 1wherein a respective thickness of the one or more regions is based on adistance between the respective region and the source of the betaparticles.
 18. The betavoltaic power source of claim 1 wherein thesource of beta particles comprises promethium-147 (Pm-147) and excludespromethium-146 (Pm-146).
 19. The betavoltaic power source of claim 1having an energy density in a range from 1 watt-hour per cubiccentimeter to 500 watt-hours per cubic centimeter, wherein the energydensity is calculated by integrating the betavoltaic power source powerdensity over a lifetime of the power source.